My Celestia. I don't know what to say to this other than "Wow". This is very gorgeous! I really like all the little details in artwork, and yours is full of nice ones! The shininess of the metal parts, and the perfection of that malevolent stare... very lovely!
I am imagine this is how it would feel suddenly standing next to this mutation of utter darkness. Do I sense myself maybe actually able to support her? I don't know but this perspective is just amazing.
The Nightmare's wings seem small compared to the imposing body, even darker than the night. Yet power pours from all around her, inside her, but that icy glare of disdain is more than enough to keep you bowing, begging for mercy at her hoof. Lest she simply step on you and grind you into the earth.
The lights, shadows and colors ale amazing. Her mane alone is great, same with the background. The perspective didn't turn out very good though. It's ok with the legs, but the rest... it's hard to explain as I'm not the best at it myself. The wings are also placed oddly, though it's more show-like. Great piece, anyway.